Since lithium-ion secondary batteries have small sizes and large capacities, they have been used in a wide range of areas, such cellular phones and notebook-size personal computers. Moreover, in recent years, they have been about to be used in hybrid vehicles and electric vehicles as well.
A lithium-ion secondary battery is constituted of a positive electrode, a negative electrode, an electrolytic solution, and a separator. The positive electrode comprises: a positive-electrode active material comprising a metallic composite oxide of lithium and transition metal, such as lithium-manganese composite oxides, lithium-cobalt composite oxides and lithium-nickel composite oxides, for instance; and a current collector covered with the positive-electrode active material.
The negative electrode is formed by covering a current collector with a negative-electrode material comprising a negative-electrode active material that enables lithium ions to be sorbed therein and desorbed therefrom. Negative-electrode active-material particles comprise a negative-electrode active material being capable of sorbing and desorbing lithium ions. Recently, it has been investigated to employ compounds, which include silicon (Si) or tin (Sn), or which include these two elements, for the particles. Negative-electrode active-material particles, which comprise silicon and tin, or which comprise a compound including the two, are expanded or contracted volumetrically by means of the sorbing and desorbing of lithium ions. A film is formed on a surface of the negative-electrode active-material particles at the time of charging, and thereby an electrolytic solution is prevented from contacting directly with the negative-electrode active-material particles so that the electrolytic solution is suppressed from deteriorating. This film, however, might possibly associate with such a case where the volumetric changes of the negative-electrode active-material particles have caused cracks to occur therein. When cracks arise in the film, there might possibly arise such a fear that the electrolytic solution has contacted directly with the negative-electrode active material, and thereby the electrolytic solution has been deteriorated, so that the resulting cyclability of charging and discharging has declined.
Conventionally, attempts have been carried out in order to upgrade battery characteristics, such as the cyclability, by adjusting the particle diameters of negative-electrode active materials that constitute the negative-electrode material. For example, Patent Literature Nos. 1, 2 and 3 disclose to upgrade the cyclability and charging/discharging characteristics of batteries by adjusting the BET specific surface area of silicon composite powders, which serve as a negative-electrode active material, within predetermined ranges.
Patent Literature No. 4 sets forth that a proportion of fine particles, which are included in a negative-electrode active-material powder and whose particle diameter is 5 μm or less, is set at 20% or less to moderately keep the contact between a conductive-additive powder and a negative-electrode active-material powder, thereby upgrading the resulting discharge capacities and initial charge and discharge capacities.
Patent Literature Nos. 5, 6 and 7 disclose to upgrade the resulting discharge capacities and cyclability by adjusting the average particle diameter (or “D50”) of a silicon oxide powder within a predetermined range.
Moreover, in recent years, investigations have been made as to the components within electrolytic solutions, and as to the grain size of negative-electrode active-material particles. Patent Literature No. 8 shows the following: using a silicon oxide as a negative-electrode active material; setting negative-electrode active-material particles' median diameter at from 5 μm or more to 200 μm or less; and adding fluoroethylene carbonate (or FEC) to an electrolytic solution. Patent Literature No. 9 shows that it is good to even carry out a classifying operation in order to set up the average particle size of a negative-electrode material so as to be 5-40 μm. Patent Literature No. 10 shows using SiO, a negative-electrode active material, which includes particles whose average particle diameter is 15 μm but which does not include any particles whose particle diameter is 5 μm or less in an amount of 10% or more. Patent Literature No. 2 shows the following: classifying SiO after pulverizing it; and using a powder whose average particle diameter is 5 μm and grain-size distribution is 1-10 μm. Patent Literature No. 11 has a such description that particles containing a silicon compound do not include practically any particles whose average particle diameter is 0.1 μm or less. Patent Literature Nos. 12, 13, 14 and 15 show adding FEC to electrolytic solutions.